The present invention relates to a differential pressure transmitter having a semiconductor sensor adapted to convert a differential pressure between pressures of a high-pressure fluid and a low-pressure fluid into an electric signal.
The semiconductor sensor of conventional differential pressure transmitter of the kind described usually has a resistance pattern formed by a diffusion process on a semiconductor substrate such as of silicon or the like. The pressure of the high-pressure fluid is transmitted to one side of the semiconductor sensor through a high-pressure side seal diaphragm and a high-pressure side fill liquid, while the other side of the semiconductor sensor receives the pressure of the low-pressure fluid through the low-pressure side seal diaphragm and the low-pressure side fill liquid. The arrangement is such that the differential pressure between the pressures of the high and low pressure fluids is converted by the resistance pattern of the semiconductor sensor into an electric signal of a level corresponding to the differential pressure.
The use of the semiconductor sensor has contributed to the improvement in the measuring precision and reliability. However, there still are the following requirements for differential pressure transmitter having stable performance and compact construction.
Firstly, it is required that organic materials such as "O" ring, flexible printed board or the like are not present in the region filled with the fill liquid.
If a flexible printed board is used for delivering the electric signal from the semiconductor sensor to the outside, the organic material of the flexible printed board is inconveniently dissolved in the fill liquid. Similarly, "O" rings in such region filled with the fill liquid is dissolved by the fill liquid. The liquid in which the organic material is dissolved contaminates or adversely affects the resistance pattern of the semiconductor sensor to degrade the quality of the electric signal transmitted from the transmitter.
Second requisite is to minimize the single-side pressure effect and the static pressure effect. Supposing that the maximum operating fluid pressure is applied alternately to the respective sides of the semiconductor sensor, to only one side of the sensor at a time, there remains an offset of the zero point after the removal of the pressure. The magnitude of this offset at each side is referred to as the single-side pressure effect. Also, the term "static pressure effect" is used herein to mean the offset of the zero point when the maximum operating pressures are applied simultaneously to both sides from the zero point under application of no pressure to both sides of the semiconductor sensor.
The third requisite is to effect a suitable overload protection; i.e., to protect the semiconductor sensor, which receives at its both sides widely varying pressures, from being subjected to damage or breakage due to excessive deflection caused by application of excessively high fluid pressure.
The fourth requisite is to constitute the pressure receiving portion and the sensor portion of the sensor from separately constructed ports separably attached together, so as to facilitate the pick-up of the electric signal and fabrication and to make the sensor as a whole compact.